1. Field of the Invention
This invention relates to a semiconductor device having a lead frame including a die attach pad with a contiguous lead which serves as a ground lead and at times a heat sink. More particularly, the invention relates to ground bond pads on the die attach pad.
2. Description of the Related Art
In prior art packages, especially those encapsulating semiconductor device die for logic, sensing and power devices, it is known to provide a ground terminal on the top surface of the die which is connected electrically through a bond wire to a lead frame die attach pad and/or an external lead. A ground lead contiguous with the die attach pad will may conduct heat from the bottom of the die to a printed wire board. Thus, the ground lead may serve the dual purposes of removing heat and also providing an electrical ground connection to the terminal at the top of the die.
It has been discovered that semiconductor device packages having a die attach pad with a contiguous ground lead that serves as a heat-sink and that has a bond wire from the top of the die to the periphery of the die attach pad, are prone to failure at a high rate after several hundred temperature cycles.
Even when the temperature coefficient of expansion of the lead frame metal and that of the solid resin body are nearly the same, changes in temperature of the resin- encapsulated metal lead frame parts occur faster than changes in the temperature of the solid resin body. This results in physical movement and displacement between peripheral parts of the lead frame die-attach pad and the surrounding bonded resin. This displacement is most severe at the joint of the die-attach pad and the bond wire, and catastrophic failure of that joint occurs after repeated temperature cycles.
A second problem occurs if the bond wire attaches to the die attach pad near the base of the die. This means that the wire must approach the bond at or near a right angle. This right angle approach amplifies accumulation of cycle induced stress and thereby encourages failure.
Both of these problems have been addressed in U.S. Pat. No. 5,289,344, hereby incorporated by reference. FIG. 1 of that patent discloses the use of an elevated bond pad wing which is largely encased by plastic and is not subject to the destructive forces of thermal cycling because there is little movement between metal and the encapsulating resin. Thus, there is little or no joint fatigue between the bond wire and bond pad wing after multiple temperature cycles.
In addition, the elevated bond pad wing is in the same plane as the top of the die thus allowing the bond wire to approach the bond pad wing at a very low (small) angle. This small approach angle minimizes accumulation of cycle induced stress and is less prone to failure.
Other semiconductor device packages such as the SOT-89 (TO-243) have a standard size. In order to accommodate larger die the die attach pad must be larger. This usually results in less space available on the die attach pad to place a ground bond pad. However, the same problems with thermal cycling and wire bond angle to the bond pad still remain.
What is needed is a semiconductor device package which will accommodate large die and still have a bond pad structure usable in small areas of a die attach pad which solve the problems of the prior art.